News Release
California Department of Food and Agriculture
Media Contact:
Steve Lyle, Office of Public Affairs, (916) 654-0462,
Release #03-074
Horse infected in San Diego County
SACRAMENTO – The California Department of Food and Agriculture is announcing a suspected case of West Nile virus (WNV), a mosquito-borne illness, in a horse in San Diego County. The horse is recovering under the care of a veterinarian. This would be the first known case in which a horse has been infected in California.

Horses are a dead-end host for WNV, meaning they cannot infect people or other animals. Mosquitoes become infected after feeding on birds that have high levels of the virus in the blood. The mosquitoes then pass the virus onto horses while feeding on them. Not every horse exposed to the virus will develop WNV.  Of the horses that do develop clinical signs, approximately 30 percent will succumb to the disease. 

 Signs of the disease in horses include stumbling, staggering, wobbling, weakness, muscle twitching or inability to stand. A veterinarian should be consulted if a horse is exhibiting these signs.    

Vaccinations for horses are available. Horse owners should contact a veterinarian to discuss this preventive measure, especially in counties where there is known WNV infection. The timing of the vaccine is critical because it must be administered prior to disease exposure.

“CDFA strongly recommends that horse owners take all precautions, including mosquito control measures, to protect the equine population,” said CDFA Secretary William (Bill) J. Lyons, Jr. “Since mosquitoes are the primary means of spreading this disease, a key to prevention is reducing exposure to mosquitoes.”

CDFA has collaborated with other state, federal and local agencies to detect and respond to the disease in California. CDFA has also taken its public education program about WNV directly to the equine community at horse-related events throughout California for several years, providing free testing and stressing mosquito control measures.

More information is available online at: and  and from local mosquito and vector control districts.

Information on human health is available at the California Department of Health Services, 866-847-2246 or



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California Department of Food and Agriculture Office of Public Affairs
1220 N St., Ste. 214, Sacramento, CA 95814